The invention relates to a surface pattern with a visually visible mosaic which includes a number of surface portions embedded in a laminate of at least a transparent cover layer and a protective layer.
Such surface patterns have a microscopically fine relief structure and are suitable as security elements for enhancing the level of security against forgery of value-bearing papers or securities and bonds, passes, payment means and other valuable articles.
A surface pattern of the kind set forth in the classifying portion of claim 1 is known from WO 87/07034. The surface pattern has three surface portions with an optically effective diffraction structure. Those structures with a spatial frequency f diffract visible light according to its wavelength x at different diffraction angles a. The profile height h of the grooves of the three structures is constant in each surface portion, but it is established differently in each surface portion in such a way that, for a given observer, the first structure diffracts blue light, the second structure green light and the third structure red light, with in each case a vanishing or minimum level of diffraction efficiency. When the surface pattern is tilted about an axis which is parallel to the grooves of the structures, the first surface portion will appear dark at a first viewing angle, the second surface portion will appear dark at a second viewing angle and the third surface portion will appear dark at a third viewing angle, that is to say from the point of view of the observer the dark surface portion abruptly changes in position as the surface pattern is continuously tilted. The spatial frequency f is limited upwardly as the optical effect described here is observable in the first diffraction order.
WO 98/26373 (which subsequently issued as U.S. Pat. No. 6,369,947) describes a surface pattern with diffraction gratings with a light-dark pattern, the extent of which changes with the viewing angle. The profile heights h of the gratings change in accordance with a modulation function. The spatial frequencies f are so selected that at least a first diffraction order occurs.
EP 0 712 012 A1 describes a surface pattern which includes an element with a diffraction structure coated with a lacquer, with a spatial frequency f of more than 2000 lines/mm. It is pointed out that, for such spatial frequencies, because of total reflection at the lacquer-air-interface, visible light diffracted at the diffraction structure, even in the first diffraction order, remains trapped in the lacquer layer. The diffraction structure is produced through a mask by means of an anisotropic etching process. The profile heights h depend on the size of the openings in the mask or the duty ratio of the transparent and opaque surfaces and are only statistically established by virtue of the etching process. Because of imponderables in the etching process a predetermined pattern cannot be precisely converted into the diffraction structure. A holographic copy of the diffraction structure has a similar diffraction performance to the original which—because it is itself imprecisely defined—scarcely differs from the copy, from the point of view of a lay person.
On the other hand EP 0105 099 B1, EP 0 330 738 B1 and EP0 375 833 B1 disclose surface patterns with surface portions which are assembled in a mosaic, with various diffraction structures which are visible in dependence on the tilt angle and/or angle of rotation and which show a sequence of patterns or images. The surface patterns which have an optical diffraction effect are embedded between layers of transparent plastic materials (Swiss patent specification No 678 835).